Bone-conducted speech: intelligibility functions and threshold force levels for spondees.

Threshold force values for spondees were obtained for thirty-six subjects with normal hearing using the Radioear B-70-AA and B-71 bone vibrators, which were coupled to the head with a Radioear P-3333 headband. Measurements were made on a Bruel & Kjaer model 4930 artificial mastoid. In addition to the 1000 Hz calibration tone preceding the spondees, speech noise and white noise were dubbed on the tape at the same level as the tone. Since the results indicated similar threshold force levels for both vibrators, the data from the B-70-AA and B-71 were pooled. These values in dB re 1 micro Newton are 53.2 dB for 1000 Hz, 56.8 dB with white noise, and 62.4 dB using speech noise. Any of these three calibrating signals appear appropriate when delivering speech to the B-70-AA or B-71 bone vibrators. Another purpose of this study was to establish normal speech intelligibility functions for the CID W-22 test using the B-71 vibrator. Sixteen subjects with normal hearing were tested at sensation levels (SLs) ranging from 0 to 34 dB. The results showed that intelligibility scores ranged from about 15 percent at 0 dB SL to 95 percent at 34 dB SL, and that the slope of the intelligibility function systematically decreased with increasing intensity. These findings are typical of speech intelligibility functions obtained via air conduction.